Search for variables in six Galactic open clusters

New Astronomy 52 (2017) 133–139

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Search for variables in six Galactic open clusters Ernst Paunzen a,∗, Gerald Handler b, Monika Lendl c, Bernhard Baumann d, Christian Rab d, Stefan Meingast d, Monika Rode-Paunzen d, Martin Netopil a, Victoria Antoci e, Liying Zhu f, Miloslav Zejda a, Hrvoje Božic´ g a

Department of Theoretical Physics and Astrophysics, Masaryk University, Kotláˇrská 2, 611 37 Brno, Czechia Nicolaus Copernicus Astronomical Center, Bartycka 18, 00-716 Warsaw, Poland c Austrian Academy of Sciences Space Research Institute, Schmiedlstraße 6, A-8042 Graz, Austria d Institut für Astrophysik, Universität Wien, Türkenschanzstraße 17, A-1180 Wien, Austria e Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University Ny Munkegade 120, DK-8000 Aarhus C, Denmark f Yunnan Astronomical Observatory, Chinese Academy of Sciences, P. O. Box 110, 650011 Kunming, China g Hvar Observatory, Faculty of Geodesy, University of Zagreb, Kacˇ i´ceva 26, HR-10000 Zagreb, Croatia b

h i g h l i g h t s • • • • •

Six Galactic open clusters (five of them younger than 40 Myr) were investigated for variable stars of all kinds. About 26 0 0 0 individual CCD frames of more than 312 h of photometry were analyzed. A detailed time-series analysis was performed resulting in 11 new variables. Several different types of variable stars, such as eclipsing binaries, pulsating, and rotational induced variables were detected. A detailed membership probability study helped to estimate the ages of these variables.

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 3 July 2016 Revised 25 October 2016 Accepted 31 October 2016 Available online 9 November 2016 Keywords: Stars: variables: general Open clusters and associations: ASCC 130 Open clusters and associations: Berkeley 4 Open clusters and associations: NGC 1893 Open clusters and associations: NGC 2244 Open clusters and associations: NGC 6830 Open clusters and associations: NGC 7296

individual: individual:

Variables in open cluster (known distance, age, and metallicity) fields play an important role in stellar astrophysics because they allow to investigate the interior of stars. Therefore, six Galactic open clusters were selected to search for new variables and to complement data for already known variables. As five of these clusters are younger than 40 Myr, we aim at finding variable high-mass stars such as β Cephei and Slowly Pulsating B-type stars as well as classical pulsating stars within the instability strip. About 26 0 0 0 images (312 h) photometric images were taken at the 0.8 m (Vienna, Austria) and 1.0 m (Hvar, Croatia) telescope using V and I filters. The differential light curves were analyzed with standard time series analysis methods. In total, 11 variables were found in all investigated clusters. For nine of them, we were able to determine their nature and period. In addition, the membership probabilities from the literature were analyzed.

individual: individual: individual: individual:

1. Introduction Variable stars play a major role in stellar astrophysics. Using the technique of asteroseismology, for example, we can study their stellar structure and evolution. Variable stars can also be used to measure distances in model-independent ways. In fact most astro∗

© 2016 Elsevier B.V. All rights reserved.

Corresponding author. Fax: +420 549 49 3305. E-mail address: [email protected] (E. Paunzen).

http://dx.doi.org/10.1016/j.newast.2016.10.012 1384-1076/© 2016 Elsevier B.V. All rights reserved.

physical processes as well as stellar formation and evolution can be tested with variable stars (Angus et al., 2015). To provide a meaningful observational base for different star groups, it is best to investigate stars that originate from the same astrophysical environment, i.e. stars in open clusters. Such an approach has several advantages. The distance, age and metallicity are, in general, not straightforward to determine for Galactic field stars. Star clusters on the other hand represent samples of objects of similar age and homogeneous chemical composition, suited, again, for the study of

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E. Paunzen et al. / New Astronomy 52 (2017) 133–139 Table 1 The parameters of the six target clusters taken from the DAML02 catalogue (Version 3.5) by Dias et al. (2002).

α (20 0 0)

Cluster ASCC 130 Berkeley 4 NGC 1893 NGC 2244 NGC 6830 NGC 7296

C0042+641 C0519+333 C0629+049 C1948+229 C2226+520

δ (20 0 0) (o ” ’)

l (h)

b (o )

D (pc)

E (B − V ) (mag)

log t

(h m s) 23 52 48 00 45 01 05 22 44 06 31 55 19 50 59 22 28 01

+62 +64 +33 +04 +23 +52

116.21 122.57 173.59 206.31 60.50 101.88

+0.34 +1.79 −1.68 −2.07 −2.16 −4.59

3400 2460 3500 1660 1640 2450

0.52 0.83 0.56 0.47 0.50 0.24

7.03 7.10 6.18 6.28 7.57 8.45

26 24 23 05 24 42 56 30 06 00 19 22

Table 2 Observation log. Cluster

JD(start) [2450 0 0 0+]

JD(end) [2450 0 0 0+]

N

filter

Cluster

JD(start) [2450 0 0 0+]

JD(end) [2450 0 0 0+]

N

filter

ASCC 130

4360.24993 4360.25091 4364.23624 4364.23722 4365.24692 4365.24791 4366.23348 4366.23447 4367.33719 4367.33817 4664.34374 4664.34296 4675.31193 4675.31115 4676.32259 4676.32205 4677.34350 4677.34271 4682.30999 4682.30921 4684.32243 4684.32165 4685.35995 4685.35917 4686.29560 4686.29482 4690.33779 4690.33701 4707.27271 4707.27193 4709.26646 4709.26568 4710.29635 4710.29557 4715.31166 4715.31087 4716.27559 4716.27481

4360.50734 4360.50834 4364.36265 4364.36197 4365.64811 4365.64713 4366.55782 4366.55684 4367.53385 4367.53278 4664.54176 4664.54091 4675.41409 4675.41323 4676.58359 4676.58274 4677.57017 4677.56988 4682.56498 4682.56413 4684.44453 4684.44482 4685.52084 4685.51999 4686.47160 4686.47070 4690.54176 4690.54090 4707.53352 4707.53296 4709.55865 4709.55780 4710.55877 4710.55792 4715.54450 4715.54365 4716.58470 4716.58440

393 381 194 191 606 602 495 490 299 269 459 351 252 188 630 477 549 417 596 454 279 217 375 290 432 322 495 368 573 437 693 523 588 450 515 390 705 534

I V I V I V I V I V I V I V I V I V I V I V I V I V I V I V I V I V I V I V

NGC 1893

4141.24031 4141.23726 4141.23897 4506.22975 4506.22876 4507.39848 4507.39782 4508.23505 4508.23430 4513.24017 4513.23950 4516.41705 4516.41641 4517.30061 4517.30026 4521.22981 4521.22910 4523.23522 4523.23462 4524.22579 4524.22519 4525.38564 4525.38503 5418.34199 5419.44419 5420.31941 5421.29931 5424.34510 5425.30794 5426.34775 5460.31875 5463.27002 5467.28986 5460.43748 5463.30648 5463.47954 5467.38800 5469.32913

4141.43300 4141.43037 4141.43167 4506.24019 4506.23920 4507.50810 4507.50750 4508.50770 4508.50696 4513.50433 4513.50363 4516.49130 4516.49193 4517.40944 4517.40909 4521.44460 4521.44483 4523.45650 4523.45583 4524.29491 4524.29428 4525.42968 4525.42802 5418.43523 5419.45789 5420.43377 5421.44146 5424.37895 5425.35012 5426.45553 5460.34925 5463.29437 5467.30816 5460.45038 5463.33134 5463.51132 5467.51455 5469.40419

129 172 129 18 18 291 291 546 549 552 546 177 179 332 335 602 649 716 716 228 228 71 93 242 21 286 365 92 113 273 50 55 40 6 22 26 94 44

I R V I V I V I V I V I V I V I V I V I V I V V V V V V V V V V V V V V V V

Berkeley 4

processes linked to stellar structure and evolution, and to fix lines or loci in several most important astrophysical diagrams such as the color-magnitude diagram, or the Hertzsprung–Russell diagram (HRD, Paunzen and Netopil, 2006). It is, therefore, natural to make use of the advantages which variable star cluster member offer. Space based missions such as CoRoT, Kepler, and MOST, with a high data cadence covering a long time-base, also made use of star clusters (Briquet et al., 2011; Gruber et al., 2012; Nardiello et al., 2015). Although ground based observations are normally less accurate than space based ones, they are still needed and valuable in order to analyze the variable star content of clusters. Such data sets could cover, for example, sky regions which are not accessible to the satellites or different magnitude ranges. In the field of variable star research, still, many questions are not answered, yet. For example, the β Cephei stars are one group of opacity-driven pulsators (Moskalik and Dziembowski, 1992). Theory predicts an instability region for these pulsators in the HRD which is not compatible with the observationally determined posi-

NGC 2244

NGC 6830

NGC 7296

tions of the β Cephei stars. However, the instability strips in clusters can be determined with good accuracy because their distance can be taken advantage of. Using open clusters, one can also examine the apparent absence of β Cephei stars at the high- and low-mass ends of their instability strip because objects lying in these regions are measured at the same time (Handler and Meingast, 2011). In this paper, we present the results of 25 775 measurements for six Galactic open clusters (ASCC 130, Berkeley 4, NGC 1893, NGC 2244, NGC 6830, and NGC 7296). In total, 312 h of photometry were analyzed. For the six cluster fields, we analyzed the light curves of nine variable stars and their membership probabilities. 2. Target selection, observations, and reduction For the target selection the DAML02 catalogue (Version 2.7) by Dias et al. (2002) and WEBDA1 (Mermilliod and Paunzen, 2003) 1

http://webda.physics.muni.cz/.

E. Paunzen et al. / New Astronomy 52 (2017) 133–139

135

were used. The clusters should be included in the extensive studies by Kharchenko et al. (2005); 2013) because they list membership probabilities based on different criteria. We selected three open clusters (ASCC 130, NGC 1893, and NGC 2244) for which variable star studies are already available. These clusters are not only used to check our methods but also to find new variables. In addition, another three open clusters (Berkeley 4, NGC 6830, and NGC 7296) were chosen for which no such studies are available, yet. The photometric observations were performed at the following two sites with the described telescopes and instruments: •

•

Hvar Observatory, University of Zagreb, 1.0 m Austrian-Croatian Telescope (ACT); Apogee Alta U47 CCD camera, 1024 × 1024 pixels, field-of-view of about 3’, Institute of Astronomy of the University of Vienna, 0.8m “vienna little telescope” (vlt); SBIG STL-6303E CCD camera, fieldof-view of 20.8’ × 13.9’.

The exposure times vary between 10 and 60 seconds depending on the used filter, weather conditions and the target field. The observation log is listed in Table 2. In total, we analyzed 25 775 images with about 312 h of photometry. The data reduction and differential photometry were performed using the C-Munipack2 package. The complete photometric data were transformed to a standard system for each filter. This is essential for any scientific analysis. Bessell (2005) summarized and discussed the problems concerning the variations of atmospheric extinction coefficients, transformation equations, different filter systems, CCDs as well as the difference between two sets of standard stars. Here, we used the method described by Haug (1980). The unknown coefficients were determined via a multivariate analysis. The published VI photometry for stars of each cluster field was taken from WEBDA. No offsets for the different nights of the individual clusters were found. The time series analysis of the differential light curves (Fig. 3) was conducted applying a standard Fourier technique and the Phase-Dispersion-Method. All computations were done within the program package Peranso3 (Paunzen and Vanmunster, 2016). The reduced light curves as well as the raw images are available upon request from the first author. Throughout this paper, we use the numbering system from WEBDA denoted as ‘W#’ for the stars in the corresponding cluster fields. 3. Results In Fig. 1, the basic diagnostic diagrams to detect variability are shown. They were generated within the C-Munipack package using the time series in Johnson V. For the clusters for which we also have observations in Johnson I, these diagrams are comparable. The correlation of the intrinsic deviation is, due to the photon noise, correlated with the apparent magnitude for non-variable stars. The statistical significance of these diagrams is comparable to the 4σ ratio of the amplitude signal/noise for the classical Fouriertechnique (Breger et al., 1993). Furthermore, we also analyzed the differential light curves of all program stars with the VARTOOLS package4 (Hartman et al., 2008). The results confirmed our previous ones of the detection of non-constant stars. All stars which are significantly (more than 3σ ) above the mean value were analyzed in more detail. For estimating the membership probabilities of the detected variable stars, we used the results by Kharchenko et al. (2013).

2 3 4

http://c-munipack.sourceforge.net/. http://www.peranso.com/. http://www.astro.princeton.edu/∼jhartman/vartools.html/.

Fig. 1. The statistical photometric errors of the observed time series. The variable stars are clearly visible as outliers.

They made a corresponding analysis for 3784 star clusters using photometry from 2MASS and kinematical data from the PPMXL catalogue (Table 3). Four different membership probabilities were derived on the basis of the location of the object according to the cluster center, the proper motion, and two photometric 2MASS diagrams. Only for one variable star, NGC 7296 - W212, no data were found in Kharchenko et al. (2013). ASCC 130: There are two known long period variables (NSV 14726 and NSV 26155) in this cluster. For NSV 14726, Weber (1959) reported a late spectral type (no further details are listed), an amplitude of 0.8 mag, and an irregular behavior. NSV 26155 is a carbon star flagged as variable (Alksnis et al., 2001). These objects show pulsations with periods from 30 to 100 days and an amplitude of about a few tenths of a magnitude. In addition, there are irregular and unpredictably sudden drops in the light curves by one to nine magnitudes. With our data set, we are not able to add any additional information for NSV 26155. For these two stars, we derived a nightly average for the five nights. For NSV 14726, we get a possible period of 227 d which has to be treated with caution. We detected one new variable star, 2MASS 23515128+6220410, which is of W Ursae Majoris type. According to Kharchenko et al. (2013), this star is not a member of ASCC 130. Berkeley 4: We found one new eclipsing binary, TYC 4024-23331 (W2002), with a period of 3.76694 d which is a real member of the cluster (Table 3). We analysed the V and I light curves in more detail. For this, we used the 2013 version of the Wilson–Devinney code (Wilson and Van Hamme, 2003). The effective temperature of the primary component (the star eclipsed at primary minimum)

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E. Paunzen et al. / New Astronomy 52 (2017) 133–139 Table 3 The published data by Kharchenko et al. (2013) of the found variable stars (Fig. 3) except NGC 7296 W212, for which no data are available. The lower panel lists the membership probabilities due to the location of the object according to the cluster center (PD ; [0;1]), the proper motion (Pμ ), and the photometric 2MASS diagrams (PJKs and PJH ).

Var W2002 W55 W139 W114 W178 W200 W154 W164 W166

Var W2002 W55 W139 W114 W178 W200 W154 W164 W166

α (20 0 0) (h)

δ (20 0 0)

23.8642445 0.7533700 5.3832130 5.3815875 6.5312603 6.5353711 6.5335015 19.8513443 19.8491938 19.8489638 Cluster

62.344742 64.389040 33.459019 33.392075 5.033092 4.971141 4.878056 23.102372 23.125999 23.125236 μα cos δ (mas yr−1 ) −4.88 +1.69 +4.44 +0.56 −0.01 +6.76 +1.45

(mas yr−1 ) −7.36 +0.30 −4.21 −2.83 +0.52 +1.79 +1.67

+2.18 +1.75

−5.15 −1.16

ASCC 130 Berkeley 4 NGC 1893 NGC 1893 NGC 2244 NGC 2244 NGC 2244 NGC 6830 NGC 6830 NGC 6830

(o )

B (mag)

V (mag)

J (mag)

σJ

16.600 11.473 17.0 0 0 12.058 7.722 8.943 8.700

15.100 11.016 16.700 12.369 7.617 8.814 8.586

13.500

11.700

μδ

σμ

13.571 10.295 13.916 11.574 7.243 8.496 8.152 11.946 10.887 12.138 D (deg) 0.1431 0.0157 0.0447 0.0403 0.0937 0.0592 0.0659 0.0187 0.0283 0.0290

0.032 0.030 0.030 0.030 0.030 0.030 0.030 0.035 0.030 0.030 PD (0,1) 0 1 1 1 1 1 1 1 1 1

(mas yr−1 ) 4.00 3.42 4.19 1.50 1.50 1.50 1.60 1.60 2.29

(mag)

σH

H (mag)

(mag)

13.161 10.212 13.478 11.582 7.220 8.438 8.095 11.145 10.644 12.012 Pμ (%) 47.47 91.43 90.68 30.09 73.10 0.07 30.81 35.64 49.20

0.037 0.030 0.030 0.030 0.034 0.046 0.034 0.035 0.030 0.030 PJKs (%) 0.00 41.37 100.00 0.29 99.77 100.00 100.00 0.00 0.62 94.31

Ks (mag) 13.034 10.204 13.288 11.536 7.250 8.489 8.119 10.855 10.586 11.979

σ Ks (mag) 0.035 0.030 0.035 0.030 0.030 0.037 0.030 0.030 0.030 0.030

PJH (%) 0.00 90.55 100.00 0.29 99.93 90.20 77.17 0.00 0.44 100.00

Table 4 Photometric solutions for Berkeley 4 - W2002. The errors in the final digits of the corresponding quantity are given in parentheses. Parameter

Value

e

0.124(1) 2.78(3) 1.0 1.0 85.1(2) 25 0 0 0 22 527(58) 0.27(1) 5.10(3) 3.34(7) 0.7034(12) 0.6987(11) 0.2083(12) 0.2095(12) 0.2105(12) 0.1464(87) 0.1475(90) 0.1512(102)

ω

g1 = g2 A1 = A2 i (deg) T1 (K) T2 (K) q

1 2

L1V L1V +L2V L1I L1I +L2I

Fig. 2. The sums of weighted square deviations  for all the assumed values of q for the light curves of Berkeley 4 - W2002.

was fixed to 25 0 0 0 K. This value is derived from the calibration of Strömgren uvbyβ photometry(Handler, 2011). The other fixed parameters are the gravity-darkening coefficients g1 = g2 = 1.0 (Lucy, 1967) and the bolometric albedo A1 = A2 = 1.0, which are corresponding to the radiative envelope of this binary system. The limbdarkening coefficients are computed automatically as functions of Teff , log g, and [M/H] based on Van Hamme (1993) limb darkening tables. The adjustable parameters are: the orbital eccentricity e, the argument of periastron in radians ω0 , phase shift φ 0 , the inclination i, the mean temperature of the star 2, T2 , the monochromatic luminosity of the star 1, L1V and L1I , and the dimensionless potentials of the star 1 and the star 2, 1 and 2 . To get a reliable mass ratio, q, the solutions for a group of assumed values of the mass ratio q from 0.1 to 1.0 are obtained. For each q, the calculation starts at mode ‘2’ (the detached mode). The sums of weighted square deviations  for all the assumed values of q are shown in Fig. 2. From this figure, we deduce that the minimum  is achieved at q = 0.31. By choosing the latter value as the ini-

r1 (pole) r1 (side) r1 (back) r2 (pole) r2 (side) r2 (back)

tial value and making q an adjustable parameter, we derived the final solution for this binary system which is listed in Table 4. Our result shows that it is the detached binary with the orbital eccentricity e = 0.124. NGC 1893: In two papers, Lata et al. (2012, 2014) presented 157 variables stars in the cluster field. They divided the variables in pre- and main-sequence objects. From their extensive list, we are only able to confirm the variables V14 (W55) and V38 (W139). However, several stars are slightly above the mean sigma values (Fig. 1). We analyzed these stars in more detail but found no statistically significant peaks. This might be caused by the relative short observing run for this cluster. NGC 2244: This young cluster with an age below 10 Myr still contains a significant amount of gas and dust. Its variable star content was extensively studied with the CoRoT and MOST satellite missions (Briquet et al., 2011; Gruber et al., 2012). Unfortunately,

E. Paunzen et al. / New Astronomy 52 (2017) 133–139

Fig. 3. The light curves of the variables found within the six cluster areas. If we were able to determine the period (listed in parenthesis), phases were calculated.

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E. Paunzen et al. / New Astronomy 52 (2017) 133–139

the hot variable stars HD 46223 and HD 46966 (Blomme et al., 2011) are not in our field of view and HD 46150 (V = 6.73 mag) is saturated in most of our frames. In Fig. 3, we present the light curves of three variables, namely HD 46149 (W114), HD 46180 (W178), and V0578 Mon (W200). All three objects are, with a high probability, members of NGC 2244. HD 46149 (W114) is an O-type binary showing light changes correlated with the rotational period (11.84 d) and also pulsation (Degroote et al., 2010). Our found period is well in line with published values and will be of importance when studying the long term behavior of this system. For the eclipsing binary system HD 46180 (W178), we were not able to observe the primary minimum. However, one of the visual components is an eclipsing binary with a period of about 3.09 d and a very small amplitude of about 0.02 mag with eclipses lasting only about two hours (Pribulla et al., 2010). It could be easily misidentified as an ellipsoidal variable. V0578 Mon (W200) is a well studied detached eclipsing binary system consisting of two early B-type stars and a period of 2.4085 d (Garcia et al., 2014). Our data set in Johnson V and I can be used in forthcoming analysis. NGC 6830: In the cluster area, we found three new variable stars, W154, W164, and W166. With a high probability, W166 is a member of NGC 6830, whereas the other stars are probably nonmembers. No proper motion for W154 was measured which is an indication that it is a background star. For estimating the reddening and spectral types of W154 and W164, we used the 2MASS colors listed in Table 3 and the reddening law, E (J − H )/E (H − Ks ) = 1.9, and intrinsic color indexes published by Straižys and Lazauskaite (20 08, 20 09). We get the following values for E (J − H ), (J − H )0 , and (H − Ks )0 ) for W154 as well as W164, respectively: [0.16,0.64,0.21] and [0, 0.24,0.06]. This implies spectral types of about M1 and F8 for these objects. W154 is a long period variable for which we are not able to derive an accurate period. According to the spectral type, W164 could be either a δ Scuti or γ Doradus variable. The period would be comparable with both groups. If we assume that it is not on the main-sequence, it could also be a Cepheid or a T Tauri variable. However, the found period is hardly compatible with those variable star groups. W166 was detected as chemically peculiar star via a photometry by Netopil et al. (2007). These stars show photometric variations explained in terms of the oblique rotator model, according to which, the period of the observed light variations is simply the rotational period due to surface abundance anomalies (Krticˇ ka et al., 2013). The detected period of 5.34 d is well in range with those found for other members of the group. NGC 7296: There is a CP star (Netopil et al., 2005) situated within the cluster area (W14) for which we tried to measure the rotational induced variability. However, we were not able get any statistically significant period. One new variable star was detected, W212 (2MASS 22280347+5217397). We are not able to establish the nature of its variability (Fig. 3). There are only H (14.157 mag) and Ks (14.094 mag) magnitudes available which result in (H − Ks ) = +0.063 mag. If it is unreddened, it would result in a spectral type of about F9. The available 2MASS data for NGC 7296 from WEBDA were taken to inspect the location of W212 in the Ks versus (H − Ks diagram. In this diagram it is located on the cluster main-sequence. Taking the cluster mean reddening (Table 1) and the reddening law E (H − Ks ) = 0.17E (B − V ) by Dutra et al. (2002), we get (H − Ks )0 = +0.022 mag which corresponds to a spectral type of A2. This would place the star in the classical δ Scuti instability strip. No data for this star are available in Kharchenko et al. (2013). 4. Conclusions and outlook Currently, we know of about 2200 cataloged Galactic open clusters (Dias et al., 2002). They are samples of stars of constant age

and homogeneous chemical composition at a certain distance from the Sun. Hence, star clusters are perfectly suited for the study of processes linked to stellar structure and evolution such as pulsation, diffusion, rotation, mass-loss, and meridional circulation, to mention a few. Another advantage to investigate variable stars in open cluster is that one gets light curves simultaneously for many different objects. However, there are not many (about 5%), compared to the total number of known ones, open clusters analyzed for their variable star context, yet. Most of them are observed from the ground, but several of them are also investigated via space based data (Zejda et al., 2012). We presented our efforts to find variables in six Galactic open clusters, observed at two different observatories. In total, 312 h of photometry were collected and the light curves of nine variable stars were analyzed in more detail. In addition, their membership probability and variability nature was investigated. At least six of them are true members of the corresponding cluster. Several different types of variables, for example, EBs, pulsating and rotational induced variables were detected. This shows one of the advantages to observe stars in the field of clusters because a large variety of objects with different spectral types are present. In the future, more open cluster have to be systematically investigated for their variable star content. Currently ongoing all sky surveys like Wide Angle Search for Planets (WASP, Pollacco et al., 2006) are normally limited by crowding because they employ aperture photometry. Therefore, long-term single telescope observations are still very much needed to explore variable star cluster members. This will become even more important when the parallax measurements from the Gaia satellite (Perryman et al., 2005) will be available. These will allow not only to determine the membership probability, but also the absolute magnitude much more accurate. Acknowledgements This research was partially supported by the Austrian Fonds zur Förderung der wissenschaftlichen Forschung under grant P20526N16. It was also supported by the grant GP14-26115P. GH acknowledges support by the Polish NCN grant 2011/01/B/ST9/05448 and 2015/18/A/ST9/00578. HB acknowledges financial support by Croatian Science Foundation under the project 6212 ‘Solar and Stellar Variability’. Stefan Meingast is a recipient of a DOC Fellowship of the Austrian Academy of Sciences at the Institute for Astrophysics, University of Vienna. This research has made use of the WEBDA database, operated at the Department of Theoretical Physics and Astrophysics of the Masaryk University. References Alksnis, A., Balklavs, A., Dzervitis, U., Eglitis, I., Paupers, O., Pundure, I., 2001. Baltic Astron. 10, 1. Angus, R., Aigrain, S., Foreman-Mackey, D., McQuillan, A., 2015. MNRAS 450, 1787. Bessell, M.S., 2005. ARA&A 43, 293. Blomme, R., Mahy, L., Catala, C., et al., 2011. A&A 533, A4. Breger, M., Stich, J., Garrido, R., et al., 1993. A&A 271, 482. Briquet, M., Aerts, C., Baglin, A., et al., 2011. A&A 527, A112. Degroote, P., Briquet, M., Auvergne, M., et al., 2010. A&A 519, A38. Dias, W.S., Alessi, B.S., Moitinho, A., Lépine, J.R.D., 2002. A&A 389, 871. Dutra, C.M., Santiago, B.X., Bica, E., 2002. A&A 381, 219. Garcia, E.V., Stassun, K.G., Pavlovski, K., Hensberge, H., Gómez Maqueo Chew, Y., Claret, A., 2014. AJ 148, 39. Gruber, D., Saio, H., Kuschnig, R., et al., 2012. MNRAS 420, 291. Handler, G., 2011. A&A 528, A148. Handler, G., Meingast, S., 2011. A&A 533, A70. Hartman, J.D., Gaudi, B.S., Holman, M.J., McLeod, B.A., Stanek, K.Z., Barranco, J.A., Pinsonneault, M.H., Kalirai, J.S., 2008. ApJ 675, 1254. Haug, U., 1980. A&A 84, 23. Kharchenko, N.V., Piskunov, A.E., Röser, S., Schilbach, E., Scholz, R.-D., 2005. A&A 438, 1163. Kharchenko, N.V., Piskunov, A.E., Schilbach, E., Röser, S., Scholz, R.-D., 2013. A&A 558, A53.

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